Oriented strandboard (OSB) is a wood-based composite used in construction, furniture-making, and other applications. OSB is made by cutting and drying wood strands, applying wax and bonding resin (binder) to the strands, forming the treated strands into a continuous mat, and consolidating the mat under heat and pressure, usually in a hot press. On average, strands in OSB are about 1/32″ thick, ½″ to 1½″ wide, and 2-6″ long. The OSB mat usually includes discrete surface and core layers. Strands in the surface layers are generally larger than those used in the core layer, and are oriented parallel to the machine direction of the mill forming line. Conversely, strands in the core layer are oriented in the cross-machine direction.
Different bonding resin types are used to make OSB: liquid phenol/formaldehyde resole resins (LPF), such as Georgia-Pacific's 70CR66 resin; powdered phenol/formaldehyde resole resins (PPF), such as Hexion's W3154N resin; and polymeric diphenylmethane diisocyanate (pMDI) resin such as Huntsman's Rubinate 1840. A resole resin is a resin made under base-catalyzed conditions with a formaldehyde-to-phenol molar ratio greater than one.
Each resin type has its own set of performance characteristics. For example, LPF resins are lower in cost, but develop bond strength slower than pMDI and are less tolerant of high moisture levels in the wood. PPF quickly develop bond strength and are more tolerant of moisture, but are more expensive than LPF, and application rates are more limited. pMDI resin also develops bond strength quickly, and the bonds tend to be stronger than equivalent levels of LPF or PPF, but pMDI is relatively expensive, often does not work well on dry strands, and tends to bond to the press platen.
Accordingly, different binder types are often used for different OSB mat layers. For instance, it is common for pMDI to be used in the core layer, which has a relatively low temperature and a high moisture content level during the hot-pressing cycle. LPF resins are often used in the surface layers where the temperature is relatively high and the moisture content is relatively low. PPF resins are used in combination with either pMDI or LPF resins to achieve an improved balance of cost and bond performance, but processing PPF resins often requires a substantial ventilation system to minimize release of powdered PF dust from production machinery.
To be suitable for OSB, liquid bonding resins must meet certain performance and application requirements. For example, typical OSB forming equipment can only accommodate resins having a viscosity lower than a certain threshold, for example about 500 centipoise (cps). In addition, other process considerations, such as resin pump size, targeted adhesive dosing rates, wood flow rates, and so forth, may further lower the threshold viscosity, for example to about 250-300 cps.
Viscosity limitations are important when a PF resin is being used as a bonding agent. In some cases an LPF resin can be heated just prior to spraying in order to reduce the viscosity, but a countervailing concern is to avoid heating the resin to the point of initiating the curing process prior to application to the strands. Another way to reduce the viscosity of an LPF resin is to increase the water content; however, this approach may result in excessive moisture levels in the OSB mat during hot-pressing, which may increase the risk of steam explosions during OSB production.
Yet another way to reduce LPF viscosity relates to its composition. LPF resins are essentially aqueous solutions of phenol/formaldehyde polymers (and oligomers), and reducing the average molecular size of the polymer (such as by reducing the average degree of polymerization of the phenol/formaldehyde adducts) correspondingly reduces the viscosity of the resin. However, if the average molecular weight of the phenol/formaldehyde adducts is too small, the LPF resin will fail to achieve adequate bond strength formation during the hot-pressing process. Achieving a balance of low viscosity, high solids content, and rapid bond strength development is a key challenge in formulating LPF resins that are targeted for use in the production of OSB.
Most LPF resins used for OSB contain urea, which is typically incorporated to reduce viscosity and consume free formaldehyde, although it does not directly contribute to bond strength development. Urea is well-suited for these functions by virtue of its reactivity, solubility, low molecular weight, low cost, availability, and favorable toxicity profile. It is quite common for urea levels in liquid PF resins for use in OSB to be about 15-40% based on the solids content of the resin. The high level of urea in liquid PF resins for use in OSB make such resins somewhat unique relative to PF resins used in other wood bonding applications.
As noted above, LPF resins used to make OSB are solutions of phenol/formaldehyde polymers (and oligomers). Such phenol/formaldehyde adducts are soluble in a sufficiently alkaline aqueous medium. In practice, this is achieved most commonly by use of sodium hydroxide, although other group 1 metal hydroxides, such as potassium hydroxide, can be used. Sodium hydroxide is a strong base which quantitatively reacts with the acidic alcohol functional group on the phenol to form a phenate sodium salt. Thus, conventional LPF resins used to make OSB are essentially composed of water, urea, and the sodium salt of phenol/formaldehyde polymers and oligomers.
Modified LPF compositions in which lignin has been incorporated into the resin have been proposed. Somewhat similar to urea, lignin has some ability to lower viscosity in an LPF resin, and can also sequester free formaldehyde (at elevated pH levels). In addition, lignin is thought to be able to directly contribute to bond strength in wood-gluing applications. The low cost and viscosity-reducing effect make lignin a suitable candidate for replacing a portion of the urea normally present in LPF resins. Normally, the bond-forming component in such resins consists of the phenol/formaldehyde adduct(s) present in such resins; as such, due to its potential contribution to bond strength development, lignin may optionally be a candidate to replace a portion of the phenol/formaldehyde adducts in such resins.
However, it has been found that many of such lignin-containing formulations tend to exhibit slow bond strength development during the OSB manufacturing process, and increase the rate at which the resulting OSB absorbs water. Lignosulfonate (sulfonated, degraded lignin), such as produced by the sulfite wood pulping process, is the predominant lignin type that has been explored for lignin-containing LPF resins, and is thought to cause fast water absorption rates in OSB.
Recently, work has also been conducted on the use of kraft lignin—such as produced by the kraft wood pulping process, and, unlike lignosulfonate, is mostly free of sulfonic acid groups—in OSB-type LPF resins. Indeed, certain types of kraft lignin have been incorporated into an LPF resin to produce a lignin-containing resin that exhibits performance similar to that of a conventional LPF resin. Examples of these latter compositions are described in the assignee's co-pending US Patent App. Pub. No. 20110245381 of Winterowd, et al.
Native lignin is a high-molecular-weight phenylpropane polymer that is present in wood at a level of about 24-35% in softwood and about 17-25% in hardwood. Native lignin in wood is not soluble in water, and one of its functions in the plant is to bond the cellulose fibers (wood cells) together. In the commercial kraft pulping process, wood chips are steeped in aqueous solutions of sodium sulfide and sodium hydroxide at elevated temperatures in order to degrade the native lignin to the point of being soluble in water. This allows for isolation of the high-value wood fibers.
The residual aqueous solution of degraded lignin, sodium carbonate, and sodium sulfate is commonly referred to as “black liquor.” Usually, the black liquor also contains various carbohydrates. Conventional black liquor has a pH value of about 13-14. The degraded lignin present in black liquor can be isolated in discrete fractions by addition of acids to lower the pH. As the pH level is decreased, there is initial precipitation of the highest molecular weight lignin compounds. These can be separated from the residual liquor by filtration. A further reduction in the pH value results in precipitation of additional lignin compounds, which have lower molecular weight than the first fraction. This second set of precipitated compounds can also be isolated by filtration. This process can be conducted multiple times to yield an array of fractions.
Acids suitable for this process include strong acids such as sulfuric acid, nitric acid, and hydrochloric acid, or weak acids such as acetic acid or carbonic acid—the latter may be created by injecting carbon dioxide into the black liquor. The use of carbon dioxide to precipitate lignin from black liquor was described as early as 1942, for example in U.S. Pat. No. 2,282,518 to Hochwalt et al. For incorporation into a PF resin for OSB, it is important to separate the degraded lignin from the other compounds in the black liquor, such as sulfate salts and carbohydrates, which can have deleterious effects on the emission potential of a binder, the strength development rate, the ultimate bond strength, or the rate at which OSB made with the resin will absorb water.
As noted above, many prior attempts to incorporate lignin and/or spent pulping liquors into various types of phenolic resins have been made. In U.S. Pat. No. 2,282,518, lignin material isolated from black liquor and substantially free of alkaline solvents and sodium organic compounds is dissolved in a phenolic body and reacted under heat with an aldehydic material in the presence of a catalyst to make a resin for mixing with fillers and utilization in molding applications. Several other attempts, and their shortcomings, are summarized in the aforementioned US20110245381 of Winterowd et al. The subject matter thereof, and that of the references discussed therein, is incorporated herein by reference.
Some of the prior art technologies are based on the use of whole black liquor or whole spent sulfite liquor. As noted above, in practice, compounds in these whole liquors exhibit deleterious effects on the performance of a phenolic binder resin for an OSB application. Also, spent sulfite liquors contain lignosulfonate, which is largely not present in kraft lignin. The sulfonic acid groups in the lignosulfonate salts, which are present in binders that are partially comprised of sulfite liquors, tend to adversely affect the performance of the OSB when it is exposed to water.
A lignin-containing LPF resin described in the aforementioned US20110245381 of Winterowd et al. is an aqueous resole having a percent solids of about 35-65%, a pH of about 8-13, a viscosity between about 50-1,000 cps, and being composed of the alkaline metal salt of phenol/formaldehyde polymers and oligomers (40-90% of the total weight of the solids in the resin), urea (5-35% of the total weight of the solids in the resin), and a mixture of degraded lignin polymer and an alkaline metal salt of the degraded lignin polymer (5-25% of the total weight of the solids in the resin). This lignin-containing LPF resin may also contain a reaction product of the phenol/formaldehyde adduct and the degraded lignin (1-90% of the total weight of the solids in the resin), and a reaction product of the urea and free formaldehyde (0.01-5.0% of the total weight of the solids in the resin). In some embodiments, the degraded lignin polymer is lignin which has been isolated as a precipitate from kraft pulping black liquor by addition of one or more acids or carbon dioxide to adjust the pH of the black liquor to a pH of 7 to 13 and washed with water to remove the contaminants.
The degraded kraft lignin utilized in the aforementioned lignin-containing LPF resin exists as a solid at the time that it is incorporated into the LPF formulation. This “solid lignin” has been isolated from kraft liquor in a manner that removes the unwanted carbohydrates and lower molecular weight lignin material. Thus, the select lignin-containing LPF resins utilize degraded kraft lignin that has molecular weight that is greater than about 1000 Da. Thus, this lignin material is compositionally different than the lignin material in whole black liquor. As previously stated, this compositional difference impacts the performance of the resulting LPF resin.
Unfortunately, if the moisture content of the solid, relatively high molecular weight kraft lignin is relatively low (about 0-4%), then the kraft lignin solid material can be “dusty,” which may create a respiratory hazard and/or a spontaneous combustion hazard during storage and transfer of the material during shipping or resin production. Conversely, if the moisture content of the solid, relatively high molecular weight kraft lignin is relatively high (8-50%), then it can be sticky and clumpy, which makes it very difficult to meter and/or transfer in a reliable, quantitative manner. Further, bulk quantities of solid, relatively high molecular weight kraft lignin powder typically have variable and inconsistent moisture content, which makes use of the material as a formulating component exceptionally problematic. Thus, there is a need to modify the form of the relatively high molecular weight kraft lignin material so that it is a uniform, consistent raw material that may be easily transported and metered in a quantitative fashion and so that it does not present a spontaneous combustion or respiratory hazard.